Human herpesvirus-8 (HHV-8), a member of the gamma subfamily of herpesviruses, is associated with malignancies including Kaposi's sarcoma, primary effusion lymphoma, and multicentric Castleman's disease. The mechanism(s) by which the virus causes cancers is not yet clear. Cellular IL-6 is up-regulated in all of these tumors and the tumor cells are responsive to cellular IL-6. IL-6 is a multifunctional cytokine, important for cell survival, proliferation, and differentiation. Strikingly, HHV-8 and several related herpesviruses encode a homologue of IL-6. Viral IL-6 was found expressed in KSHV-related tumor cells. Viral IL-6 was able to activate the Jak/STAT pathway and support the proliferation of both human and murine B cells in vitro. However, interleukins usually function in the context of a complicated network in the body. Such complicated in vivo interactions cannot be reproduced well in vitro. Furthermore, there is no effective in vitro lytic replication for KSHV. Therefore, this application proposes to use MHV-68 as a model system to define the role of viral and cellular IL-6 in viral replication and related pathogenesis. MHV-68 is closely related to HHV-8 but does not encode an IL-6 homologue. A viral IL-6 expression cassette will be inserted into MHV-68. The function of viral IL-6 during viral replication in vitro and in vivo will be assessed and the underlying mechanism will be explored. The available knock-out mice will allow us to study the contributions of cellular factors in viral replication in vivo. This proposal, on the basis of our preliminary data, is to test the hypothesis that acquiring interleukin-6 onto the viral genome offers an advantage for viral replication and will aid us in understanding the role of cellular IL-6 and viral IL-6 in related pathogenesis. MHV-68 can form plaques on fibroblast cells in culture and infect mice in a controllable manner, which offers a unique experimental advantage. Our continuous study will be critical to reveal the function of cellular and viral IL-6, a pair of intriguing and important molecules in the life cycle of KSHV.